H&A vs non-H&A--is there a visual difference?

[slg47]

thanks yssie there is a lot to learn. As CCL mentioned a whole page on obstruction could be really interesting/useful also.

 

[yssie]

I find the opinions expressed on PS over the years can be distilled down to "SuperIdeal HA Brand" vendors versus those who only sell non branded generic rounds. The former usually make comments justifying the price premium for their precision cut or carefully selected branded HA, the latter argue that the premium is not justified and their are no discernable differences.

I beleive both side can be "correct" and it really depends on which two diamonds are being compared on a case by case basis. Whether you side with either camp the greatest emphasis should still be placed on the choice of proper CA/PA and LGF combination with recognition that that grading reports information is an average of 8 values with severe rounding (especially in the case of LGF% which is in 5% increments for GIA) and no mention of precision which makes any comparisons between branded vs unbranded only a rough estimate if only comparing data from a grading report.

CCL

Would be interesting to see a 'pepsi' test done w/ some of WF's ES near-H&A and their ACAs (though I'm quite sure of what the results would be!) both are branded, both brands carry a premium, both are (usually) well-cut to the same Tolk 'make' - or, there are enough samples present to make an interesting, if not statistically significant, comparison, and we have/are able to get light reflector info on all the stones

 

[ChunkyCushionLover]

I find the opinions expressed on PS over the years can be distilled down to "SuperIdeal HA Brand" vendors versus those who only sell non branded generic rounds. The former usually make comments justifying the price premium for their precision cut or carefully selected branded HA, the latter argue that the premium is not justified and their are no discernable differences.

I beleive both side can be "correct" and it really depends on which two diamonds are being compared on a case by case basis. Whether you side with either camp the greatest emphasis should still be placed on the choice of proper CA/PA and LGF combination with recognition that that grading reports information is an average of 8 values with severe rounding (especially in the case of LGF% which is in 5% increments for GIA) and no mention of precision which makes any comparisons between branded vs unbranded only a rough estimate if only comparing data from a grading report.

CCL

Would be interesting to see a 'pepsi' test done w/ some of WF's ES near-H&A and their ACAs (though I'm quite sure of what the results would be!) both are branded, both brands carry a premium, both are (usually) well-cut to the same Tolk 'make' - or, there are enough samples present to make an interesting, if not statistically significant, comparison, and we have/are able to get light reflector info on all the stones

If WF is willing to help participate in such a study by providing sarin scan and .gem files for potential candidates and choosing ones that missed ACA due to optical symmetry as opposed to for other reasons you might have something there.

 

[ChunkyCushionLover]

Slg47,

The "best" proportions is a bit subjective.

Choosing within the pinnacle of GIA Excellent/AGS Ideal is a safe way to screen virtual stones. The AGS-PGS and GIA charts found at http://www.octonus.com/oct/mss/gia-agspgs.phtml are useful.

These ranges will be amongst the safest even with a lack of precision:
Something like PA40.8 +/-0.1, CA34.5 +/- 0.5 and LGF 80% +/- 5% Table 53 - 57%.

Proportions close to the classical Tolkowsky still have the most balanced performance across all lighting environments.
It is not surprising most of the stones from the branded H&A PS vendors fall within these specifications.

Within the above PA/CA combinations the choice of LGF in generics is open to consumer preference and how thick they like their arrows.

Regarding LGF's:

1) Karl's article is a good one but it focusses on the singular case of Steep Crown(CA=35) with Deep Pavilion(PA=41.25) where lengthening the LGFs from (80% to 83%) helps reduce leakage under the table. However that still doesn't make the diamond with PA 41.25 equivalent in performance to an Ideal Tolkowsky.

2) Shallow Crown(like CA33.0), Moderately Deep Pavilion(PA 41.0) benefits from shorter LGF (75%) not longer LGFs see my post here.

3) My favourite round diamond (not commericially available but the AVR is close) has a steep crown and deep pavilion but a really small table and really short LGFs (55%).


Overall due to both a lack of precision on grading reports, rounding, and the complex interplay between several proportions I reccomend reflector technology and video over attempts to screen diamonds by the numbers.

 

[Karl_K]

Slg47,



Regarding LGF's:

1) Karl's article is a good one but it focusses on the singular case of Steep Crown(CA=35) with Deep Pavilion(PA=41.25) where lengthening the LGFs from (80% to 83%) helps reduce leakage under the table. However that still doesn't make the diamond with PA 41.25 equivalent in performance to an Ideal Tolkowsky.

It applies to any crown angle, the pavilion/lgf angles just change a little bit.
In what ways is it not equal? (btw I agree but I want to hear your reasons)

 

[yssie]

Slg47,



Regarding LGF's:

1) Karl's article is a good one but it focusses on the singular case of Steep Crown(CA=35) with Deep Pavilion(PA=41.25) where lengthening the LGFs from (80% to 83%) helps reduce leakage under the table. However that still doesn't make the diamond with PA 41.25 equivalent in performance to an Ideal Tolkowsky.

It applies to any crown angle, the pavilion/lgf angles just change a little bit.
In what ways is it not equal? (btw I agree but I want to hear your reasons)

I'm not ccl obviously, but if I can play here too, my thinking - a 'true tolk type' will show a greater variety of light return - colour and white - in any given lighting environment than a non-leaky steep-pav/long-lgf combo.

Because A) when increase lgf length, lower girdle facets take up a much larger surface area comparatively -that is reflecting back into the stone at the lgf angle, and B) in addition to more incoming light being reflected and refracted at that lgf angle because of sheer surface area, the angle difference between the mains and lgfs is reduced, meaning that the few rays that do bounce off the mains rather than off the lgf surface are not being internally reflected at a significantly different angle, thus for one ray that reflects off a main and another that reflects off say an adjacent lgf, output locations and output lightray angles will not be as different as they would have been had those initial reflecting surfaces been more different in angle

from a not-to-scale paintjob quickie -

 

[Karl_K]

Yssie, very good, you hit on part of the reason lets see what ccl comes up with then I will post some more.
You did hit on one key area and I will give you and ccl a hint, its about where the diamond draws light not so much where it returns it.

 

[yssie]

haha okay, I will keep it rolling 'round in my pretty lil' head until CCL returns

 

[ChunkyCushionLover]

Yssie, very good, you hit on part of the reason lets see what ccl comes up with then I will post some more.
You did hit on one key area and I will give you and ccl a hint, its about where the diamond draws light not so much where it returns it.

Karl,

Your article would have been more clear with ASET images instead of Idealscope. That is why I will always prefer the additional information provided by ASET images over Idealscope which differentiate more clearly between high angle and low angle sources of illumination.

Put very simply at an angle of 42.24 degrees the LGFs thast reside under the table although now capable of returning light can only return low angle light which is known to be of lower intensity and frequency. In addition the leakage is not totally removed just reduced.

A Classical Tolk is superior in brightness and light return capability over these proportions.

 

[ChunkyCushionLover]

What is also non equivalent about this combination is the Intensity Weighted Disperision in DC in the region of reduced light return.
But does it correlate to significantly enhanced fire over a Modern tolk round?
(I'm not sure and the relative intensity regions don't match compared to some AGSL diagrams)

 

[Andelain]

Cut consistency is not about picture-perfect H&A, yssie -- in that respect I agree with you (in fact, I think it could be argued that Infinity
I'm hoping Andelain sees this thread, because she owns two 8-stars, one of which is an I2 clarity, and the other a VS clarity, so I'd be interested to hear her perspective on owning two stones from the same brand with very different clarity grades. Does she notice a difference in performance that she can ascribe to clarity.

Hey Lula, I just spotted this post. Hope the answer is still relevant for you. This is a really intersesting comparison because Eightstar cuts their stones to such tight specifications. That Sarin reports on my stones are almost identical, excpt for the size measurements. For the record, my Eightstars are a 1.57 ct I-VVS1 and a 1.10ct F-I2. In defense on the eye-deuce, it's misgraded. There's no way in hell that should have gotten below an I-1. On a good day it might have gotten an SI-2, but not likely. But still a long way from a VVS-1. Is there a dif that I can attribute to clarity, absolutely. The I-2 is a beautiful stone, superb cut, performs every bit like a super-ideal should. The VVS-1? It has a transparency that makes me think of ripples on the clearest water, and you can just see the perfection of cut in every facet. That's the one place the I-2 isn't as good. Because of the inclusions, even though my eyes can't actually make them out, I can't quite see the absolutely perfect patterning and optical symetry of the cut that I know is there. I can see it with a loupe or if the light hits it right, but not every time like with the nearly flawless one. Sometimes it's just a garden variety super-ideal. Not that that's anything to be sneezed at, it's still gorgeous.

Hope this helps.